Could someone explain how Blender output colors
Well, solved ! :)

Thanks to the user @cegaton !
Here is what he wrote to comment 1st answer:


  @m.ardito The important information you left out
  of the picture. What you are showing are the scene referred values in
  32bit float:The compositor works with scene referred values: unbound
  linear data. Data is then processed through a display transform
  (determined in the Color Management section) and converted to Display
  referred data (in a scale from 0 to 1) The default transform is to
  sRGB and is not a linear transform. That is true for images saved in 8
  bits or 16 bit. The only format that gets saved bypassing the display
  transform is OpenEXR


So after i set Properties editor ‣ Scene ‣ Color ‣ Display Device to none, all looks correct. Value 102 of 255 is 0.4.

Thank for the help you both @cegaton and @m.ardito !